In the field of magnetic recording using a head and a medium, further improvement in performance of the magnetic recording medium and the magnetic head is demanded as the magnetic disc device has higher recording densities.
A magnetic recording medium is a discontinuous medium consisting of a set of magnetic grains, and each magnetic grain has a single-domain structure. In the magnetic recording medium, one recording bit consists of a plurality of magnetic grains. Therefore, in order to increase the recording density, the magnetic grains must be smaller, and the borders between adjacent recording bits must be less uneven. However, a problem is that as the magnetic particles become smaller, the magnetic grains are less thermally stable in association with their reduced volume.
A presumable measure to solve the above problem is to increase the magnetic anisotropy energy Ku of the magnetic grains. However, the increase in the Ku leads to an increase in the anisotropic magnetic field (magnetic coercive force) of the magnetic recording medium. On the other hand, the upper limit of the recording magnetic field intensity of a magnetic head is largely determined by the saturation magnetic flux density of the soft magnetic material configuring the magnetic core within the head. Therefore, if the anisotropic magnetic field of the magnetic recording medium exceeds an acceptable value determined based on the upper limit of the recording magnetic field intensity, recording on the magnetic recording medium fails.
Currently, a method of solving the above problem of thermal stability has been proposed in which, energy assisted recording in which assistive energy is provided to a medium during recording so as to lower the effective recording magnetic field intensity for a magnetic recording medium formed by a high Ku magnetic material. The recording system using a microwave magnetic field as the assistive energy source is called microwave assisted magnetic recording (MAMR) and is being proactively developed for practical use.
In the microwave assisted magnetic recording, a microwave magnetic field of a frequency corresponding to the effective magnetic field (Heff) for magnetization of the recording layer of a magnetic recording medium is applied in the medium in-plane direction, whereby magnetization precession is excited in the recording layer, and the recording capability of the magnetic head is assisted.
As an exemplary magnetic head using the microwave assisted magnetic recording system, a magnetic head shown in FIG. 11, is proposed that includes a main magnetic pole 6′ generating a recording magnetic field to apply to a magnetic recording medium 100′, a trailing shield 7′, and a spin torque oscillator (STO) 10′ provided between them (in the write gap) and having a multilayer structure of magnetic films (U.S. Pat. No. 9,001,465). With a current applied along the lamination direction of the spin torque oscillator 10′, the spin torque oscillator 10′ generates a microwave magnetic field through its own oscillation. With the microwave magnetic field and recording magnetic field applied to the magnetic recording medium 100′ in a superimposed manner, magnetization precession is induced in the recording layer, and the perpendicular magnetization of the recording layer is inverted. The spin torque oscillator 10′ has a first end face 10a′ forming a part of the air bearing surface, a second end face 10b′ adjacent to a trailing side end face 6b′ of the main magnetic pole 6′, and a third end face 10c′ facing the first end face 10a′. The angle θ1′ made by the first end face 10a′ and the second end face 10b′ and the angle θ2′ made by the second end face 10b,′ and the third end face 10c,′ satisfy the relationship expression θ1′≧θ2′. In such a magnetic head, in order to yield a sufficient assist effect for the spin torque oscillator 10′, the main magnetic pole 6′ and the spin torque oscillator 10′ must be close to each other. However, as the main magnetic pole 6′ and the spin torque oscillator 10′ are close to each other, a magnetic field is applied to the spin torque oscillator 10′ from the main magnetic pole 6′ during writing, whereby the spin torque oscillator 10′ oscillates unevenly, and the oscillation frequency widely fluctuates.
When the spin torque oscillator 10′ oscillates unevenly and the oscillation frequency widely fluctuates (the spin torque oscillator 10′ has a broad oscillation spectrum and a low peak value), the average value of magnetization components in the in-plane direction (the average in-plane magnetization value) of the magnetic field generation layer generating a microwave magnetic field in the spin torque oscillator 10′ is lowered. As a result, the magnetic field intensity of the microwave magnetic field generated from the spin torque oscillator 10′ becomes lower, and thus sufficient assist effect cannot be obtained.